What is the definition of Ghunnah and Ikhfah? What is the difference between them

Question
What is the definition of Ghunnah and Ikhfah. What is the difference between them with examples. In Tajweed books sometime it is written to do Ikhfah and sometime they tell to do Ghunnah, is it one of the same or there is a difference in that.
Jazakallah,